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13:00-15:00 Session 2A: Superposition and Saturation-Based Theorem Proving
Subsumption Demodulation in First-Order Theorem Proving

ABSTRACT. Motivated by applications of first-order theorem proving to software analysis, we introduce a new inference rule, called subsumption demodulation, to improve support for reasoning with conditional equalities in superposition-based theorem proving. We show that subsumption demodulation is a simplification rule that does not require radical changes to the underlying superposition calculus. We implemented subsumption demodulation in the theorem prover Vampire, by extending Vampire with a new clause index and adapting its multi-literal matching component. Our experiments, using the TPTP and SMT-LIB repositories, show that subsumption demodulation in Vampire can solve many new problems that could so far not be solved by state-of-the-art reasoners.

A Knuth-Bendix-Like Ordering for Orienting Combinator Equations

ABSTRACT. We extend the graceful higher-order basic Knuth-Bendix order (KBO) of Blanchette et al. to an ordering that orients combinator equations left-to-right. The resultant ordering is highly suited to parameterising the first-order superposition calculus when dealing with the theory of higher-order logic, as it prevents inferences between the combinator axioms. We prove a number of desirable properties about the ordering including it having the subterm property for ground terms, being transitive and being well-founded. The ordering fails to be a reduction ordering as it lacks compatibility with certain contexts. We provide an intuition of why this need not be an obstacle when using it to parameterise superposition.

A Comprehensive Framework for Saturation Theorem Proving

ABSTRACT. We present a framework for formal refutational completeness proofs of abstract provers that implement saturation calculi, such as ordered resolution or superposition. The framework relies on modular extensions of lifted redundancy criteria. It permits us to extend redundancy criteria so that they cover subsumption, and also to model entire prover architectures in such a way that the static refutational completeness of a calculus immediately implies the dynamic refutational completeness of, e.g., an Otter or DISCOUNT loop prover implementing the calculus. Our framework is mechanized in Isabelle/HOL.

A Combinator-Based Superposition Calculus for Higher-Order Logic

ABSTRACT. We present a refutationally complete superposition calculus for a version of higher-order logic (HOL) based on the combinatory calculus. We also introduce a novel method of dealing with extensionality. The calculus was implemented in the Vampire theorem prover and we test its performance against other leading higher-order provers. The results suggests that the method is competitive.

13:00-15:00 Session 2B: Interactive Theorem Proving
Competing Inheritance Paths in Dependent Type Theory: a Case Study in Functional Analysis

ABSTRACT. This paper discusses the design of a hierarchy of structures which combine linear algebra with concepts related to limits, like topology and norms, in dependent type theory. This hierarchy is the backbone of a new library of formalized classical analysis, for the Coq proof assistant. It extends the Mathematical Components library, geared towards algebra, with topics in analysis. Issues of a more general nature related to the inheritance of poorer structures from richer ones arise due to this combination. We present and discuss a solution, coined forgetful inheritance, based on packed classes and unification hints.

Quotients of Bounded Natural Functors

ABSTRACT. The functorial structure of type constructors is the foundation for many definition and proof principles in higher-order logic (HOL). For example, inductive and coinductive datatypes can be built modularly from so-called bounded natural functors (BNFs), a class of particularly well-behaved type constructors. Composition, fixpoints, and---under certain conditions---subtypes are known to preserve the BNF structure. In this paper, we tackle the preservation question for quotients, the last important principle for introducing new types in HOL. We identify sufficient conditions under which a quotient inherits the BNF structure from its underlying type. We implement in the Isabelle proof assistant a command that automates the registration of a quotient type as a BNF by lifting the underlying type's BNF structure. We demonstrate the command's usefulness through several case studies.

Extensible Extraction of Efficient Imperative Programs with Foreign Functions, Manually Managed Memory, and Proofs

ABSTRACT. We present an original approach to sound program extraction in a proof assistant, using syntax-driven automation to derive correct-by-construction imperative programs from nondeterministic functional source code. Our approach does not require committing to a single inflexible compilation strategy and instead makes it straightforward to create domain-specific code translators. In addition to a small set of core definitions, our framework is a large, user-extensible collection of compilation rules each phrased to handle specific language constructs, code patterns, or data manipulations. By mixing and matching these pieces of logic, users can easily tailor extraction to their own domains and programs, getting maximum performance and ensuring correctness of the resulting assembly code.

Using this approach, we complete the first proof-generating pipeline that goes automatically from high-level specifications to assembly code. In our main case study, the original specifications are phrased to resemble SQL-style queries, while the final assembly code does manual memory management, calls out to foreign data structures and functions, and is suitable to deploy on resource-constrained platforms. The pipeline runs entirely within the Coq proof assistant, leading to final, linked assembly code with overall full-functional-correctness proofs in separation logic.

Beyond Notations: Hygienic Macro Expansion for Theorem Proving Languages

ABSTRACT. In interactive theorem provers (ITPs), extensible syntax is not only crucial to lower the cognitive burden of manipulating complex mathematical objects, but plays a critical role in developing reusable ab- stractions in libraries. Most ITPs support such extensions in the form of restrictive “syntax sugar” substitutions and other ad hoc mechanisms, which are too rudimentary to support many desirable abstractions. As a result, libraries are littered with unnecessary redundancy. Tactic lan- guages in these systems are plagued by a seemingly unrelated issue: ac- cidental name capture, which often produces unexpected and counterin- tuitive behavior. We take ideas from the Scheme family of programming languages and solve these two problems simultaneously by proposing a novel hygienic macro system for ITPs. We further describe how our ap- proach can be extended to cover type-directed macro expansion resulting in a single, uniform system offering multiple abstraction levels that range from supporting simplest syntax sugars to elaboration of formerly baked- in syntax. We have implemented our new macro system and integrated it into the upcoming version (v4) of the Lean theorem prover. Despite its expressivity, the macro system is simple enough that it can easily be integrated into other systems.

16:45-17:45 Session 4A: Machine Learning and Theory Reasoning
ENIGMA Anonymous: Symbol-Independent Inference Guiding Machine (system description)

ABSTRACT. We describe an implementation of gradient boosted and neural guidance of saturation-style automated theorem provers that does not depend on consistent symbol names across problems. For the gradient-boosted guidance, we manually create abstracted features by considering arity-based encodings of formulas. For the neural guidance, we use symbol-independent graph neural networks and their embedding of the terms and clauses. The two methods are efficiently implemented in the E prover and its ENIGMA learning-guided framework and evaluated on the MPTP large-theory benchmark. Both methods are shown to achieve comparable real-time performance to state-of-the-art symbol-based methods.

Prolog Technology Reinforcement Learning Prover (system description)

ABSTRACT. We present a reinforcement learning toolkit for experiments with guiding automated theorem proving in the connection calculus. The core of the toolkit is a compact and easy to extend Prolog-based automated theorem prover called plCoP. plCoP builds on the leanCoP Prolog implementation and adds learning-guided Monte Carlo Tree Search as done in the rlCoP system. Other components include a Python interface between plCoP and machine learners, and an external proof checker that verifies the validity of the plCoP proofs. The toolkit is evaluated on two benchmarks and we demonstrate its extendability by two additions: (1) the learned guidance is extended to reduction steps and (2) the standard leanCoP calculus is extended with rewrite steps and their learned guidance. We show that the code size compares favorably to rlCoP and argue that the Prolog setting is suitable for combining statistical and symbolic learning methods. The complete toolkit is publicly released.

Layered Clause Selection for Theory Reasoning (short paper)

ABSTRACT. Explicit theory axioms are added by a saturation-based theorem prover as one of the techniques for supporting theory reasoning. While simple and effective, adding theory axioms can also pollute the search space with many irrelevant consequences. As a result, the prover often gets lost in parts of the search space where the chance to find a proof is low. In this paper we describe a new strategy for controlling the amount of reasoning with explicit theory axioms. The strategy refines a recently proposed two-layer-queue clause selection and combines it with a heuristical measure of the amount of theory reasoning in the derivation of a clause. We implemented the new strategy in the automatic theorem prover Vampire and present an evaluation showing that our work dramatically improves the state-of-the-art clause-selection strategy in the presence of theory axioms.

Make E Smart Again (short paper)

ABSTRACT. In this work in progress we demonstrate a new use-case for the ENIGMA system. The ENIGMA system using the XGBoost implementation of gradient boosted decision trees has shown high capability to learn to guide the E theorem prover's inferences in real time.

In this work we strip E to the barebones: we replace the KBO6 term ordering with an identity relation as the minimal possible ordering, disable literal selection, and replace evolved strategies with the simple combination of the clause weight and FIFO (first in first out) evaluatino functions.

We experimentally demonstrate that ENIGMA can learn to guide E as well as the smart, evolved strategies even without these staple automated theorem prover functionalities. To do this we need to experiment with XGBoost's meta-parameters over a dozen loops.

16:45-18:00 Session 4B: Formalizing Non Classical Logics
Mechanised Modal Model Theory

ABSTRACT. In this paper, we discuss the mechanisation of some fundamental propositional modal model theory. The focus on models is novel: previous work in mechanisations of modal logic have centered on proof systems and applications in model-checking. We have formalised a number of fundamental results from the first two chapters of a standard textbook (Blackburn et al.). Among others, one important result, the Van Benthem characterisation theorem, characterises the connection between modal logic and first order logic. This latter captures the desired saturation property of ultraproduct models on countably incomplete ultrafilters.

Formalizing a Seligman-Style Tableau System for Hybrid Logic (short paper)

ABSTRACT. Hybrid logic is modal logic enriched with names for worlds. We formalize soundness and completeness proofs for a Seligman-style tableau system for hybrid logic in the proof assistant Isabelle/HOL. The formalization shows how to lift certain rule restrictions, thereby simplifying the original un-formalized proof. Moreover, the completeness proof we formalize is synthetic which suggests we can extend this work to prove a wider range of results about hybrid logic.

A Formally Verified, Optimized Monitor for Metric First-Order Dynamic Logic

ABSTRACT. Runtime monitors for rich specification languages are sophisticated algorithms, especially when they are heavily optimized. To gain trust in them and safely explore the space of possible optimizations, it is important to verify the monitors themselves. We describe the development and correctness proof in Isabelle/HOL of a monitor for metric first-order dynamic logic. This monitor significantly extends previous work on formally verified monitors by supporting aggregations, regular expressions (the dynamic part), and optimizations including multi-way joins adopted from databases and a new sliding window algorithm.